In recent years, with remarkable development in technologies, recording intensities are increasing rapidly for optical memory devices represented by Blu-ray Disks (BDs), magneto-optical disks, etc., and magnetic memory devices represented by hard disks, etc. As an example of high-density magnetic recording/reproducing techniques, a heat assisted magnetic recording/reproducing method is known. Japanese Laid-Open patent Japanese Unexamined Patent Publication No. 4-176034/1992 (Tokukaihei 4-176034, published on Jun. 23, 1992) discloses an example of such heat assisted magnetic recording/reproducing method. This publication discloses a magnetic recording medium adopting a magnetic layer made of an N-type ferrimagnetic material having a compensation temperature (magnetic compensation temperature) at around room temperature, and also discloses the heat assisted magnetic recording/reproducing method for recording and reproducing on and from such magnetic recording medium using a laser beam (hereinafter referred to as the first prior art technique).
In the foregoing heat assisted magnetic recording/reproducing method, a recording operation is performed by heating a recording area of the magnetic recording medium with an application of a laser beam to reduce a coercivity of the recording area to a sufficiently low level, and information is then recorded in the recording area with an application of an external magnetic field by a recording magnetic head. In this recording method, the recording bit (recording marks) forming area is limited to an area where a laser beam application area and a magnetic field application area are overlapped.
The positional relationship of these areas will be explained in reference to FIG. 6. As illustrated in FIG. 6, a recording area 63 is an area where a) an area 61 having applied thereto a magnetic field by a magnetic head and b) a heated area (an area corresponding to a light spot) 62 with an application of a laser beam are overlapped, and recording bits are formed in the recording area 63. With this structure, it is possible to record a track 64 of the same width as a laser beam spot diameter yet smaller than the width of the magnetic field application area 61 (diameter of the heated area 62: 0.5 μm or smaller) on the magnetic recording medium by a magnetic recording head having a width of few μm (which is the same width as that of a conventional magnetic recording head).
When reproducing, information are reproduced from a reproducing area of the magnetic recording medium, which is heated with an application of a laser beam to increase an intensity of a residual magnetization. Here, the reproducing area is also limited to the area where the laser application area and the reproducing head area are overlapped. With this structure, it is possible to reproduce tracks recorded at a small track pitch while suppressing crosstalk.
As described, in order to realize a high density recording/reproducing, the heat assisted magnetic recording/reproducing method of the first prior art technique is characterized by reducing the recording track width while suppressing crosstalk by selectively heating a area smaller than the magnetic field application area using a laser beam as a light source.
In the above magnetic recording medium adopted in the foregoing heat assisted magnetic recording/reproducing method, an underlayer is not formed on a disk substrate, or an aluminum nitride (AlN) layer is formed on the disk substrate as an underlayer in a thickness of 60 nm, and the magnetic layer or the protective layer are formed on the disk substrate or the AlN underlayer in this order. Here, the AlN underlayer serves to improve an absorption ratio of light incident on the magnetic recording medium (i.e., the ratio of the light absorbed in the magnetic layer), which, in turn, increases the recording density.
Japanese Laid-Open patent Japanese Unexamined Patent Publication No. 315310/2000 (Tokukai 2000-315310, published on Nov. 14, 2000) discloses a technique which realize a high density recording utilizing the pinning effect by adopting NiP as a material for the underlayer, which serves to increase Ra on the surface of the underlayer (hereinafter referred to as a second prior art technique).
The pinning effect indicates such effect of hindering of motion of dislocations of a magnetic wall in a magnetic material by locally introducing therein impurities or defects, resulting in a large energy barrier being imposed against the motion of the dislocations of the magnetic wall. Here, the impurities, defects, etc., thus locally introduced are called “Pinning Site”.
In the information recording medium, the more the minimum recording bit length (the minimum recording bit length corresponding to 1 bit information in the track direction, as denoted by “M” in FIG. 6) is reduced by increasing a recording frequency (magnetic field application frequency in the magnetic field modulation method), the more the recording density can be increased.
However, in the foregoing heat assisted magnetic recording method of the first prior art technique, the magnetic recording medium adopted in the method does not provide sufficient level of recording/reproducing performances, and it is therefore difficult to form a recording bit with a minimum recording bit length of 200 nm or smaller. As a result, possible improvements in recording density are limited as is clear from the following phenomenon.
Namely, evaluation results of the recording/reproducing performances of the magnetic recording medium adopted in the above publication show that a signal quality deteriorates sharply when the minimum recording bit length is set to around 200 or less.
Further, the recording bits formed on the magnetic recording medium are observed by a Magnetic Force Microscope (MFM). The observation results show such phenomenon of respective recording bits being disturbed as being attracted to each other, or some of them being disappeared, etc., appear when the minimum recording bit length is reduced almost to 200 nm. The foregoing phenomenon can be recognized as a reduction in a track width, and the phenomenon then indicates that the track width is reduced gradually and will be interrupted eventually.
In view of the foregoing, for the conventional magnetic recording medium, a minimum recording bit length for practical use cannot be reduced further from 250 nm to ensure the reliability in its application of the heat assisted magnetic recording device.
The exchange interaction can be one of the reasons which make the recording bit shape unstable in the conventional magnetic recording medium, as will be explained below.
The smaller is a recording bit, the greater is the effect of the exchange interaction on the recording bit. Specifically, in the case of adopting the magnetic layer made of an N-type ferrimagnetic material having a compensation point at around room temperature such as a TbFeCo magnetic material, etc., the exchange interaction is exerted in the direction of aligning the magnetic direction of adjacent recording bits in one direction. In particular, in the heat assisted recording/reproducing method in which the recording area is heated, the magnetic anisotropy (coercive force in the recording/reproducing area is reduced significantly when recording, and the effect on the exchange interaction on the recording bit out of the total effects becomes larger. Therefore, when forming smaller recording bits, due to the effects of magnetizations in the surrounding, the magnetic wall is liable to dislocate, and the stable shape of the recording bits cannot be ensured. Therefore, with the foregoing first conventional technique, in order to form the recording bits with a length of not more than 200 nm, any means for suppressing the magnetic wall dislocations is necessary.
On the other hand, in the second prior art technique, a nickel phosphorus (NiP) layer is adopted as a base layer to realize high density recording. As a result, protrusions and recessions are formed on the surface of the underlayer, which serve as the pinning site, thereby realizing a high density recording.
However, NiP has a low layer separation temperature of around 350° C. (temperature at which NiP is separated into Ni and P). Therefore, when heating with an application of a laser beam in the optical assisted recording/reproducing area, not only the problem of the deformation of the recording bits but also the problem of the NiP layer separation are liable to occur. As a result, Ni deposition occurs randomly in the recording/reproducing area, which causes the deformation of the soft magnetic material. Furthermore, the deformation of the recording bits is irreversible, and does not accord with the position or shape of the magnetic bits of the magnetic layer. Therefore, such deformation can be a cause of generating noise in reproducing signals. For the reasons set for above, the NiP layer is not suited for the optical assisted recording/reproducing method.